Modern DC to DC converters and switching power supplies usually use commercial Pulse Width Modulator Integrated Circuits (PWM ICs) to significantly reduce circuit complexity and total parts count. These PWM ICs generally include a soft-start feature that slowly charges an external capacitor to control the start-up rate of the switching power supply. Under various fault conditions, the PWM IC may discharge the soft-start capacitor and initialize a power supply restart to protect both the power supply and load from overstress caused by the fault. This discharge mechanism makes commercial PWM ICs unsuitable for space applications (i.e., satellites, planetary probes, International Space Station, etc.) because of the potential for Single Event Upset (SEU).
SEU is caused by energetic particles commonly encountered in the space environment. These energetic particles can inadvertently turn on semiconductor junctions and cause an undesirable change in the operation of electronic devices and systems. An SEU of the PWM IC soft-start circuit can cause temporary interruption of the power supply output, thus affecting any electronic systems that comprise the power supply load. Therefore, many power supplies and DC to DC converters intended for use in space applications utilize PWM circuits composed of many discrete components arranged in such a way as to eliminate the possibility of SEU. This increases the total component count and circuit complexity relative to a comparable design utilizing a PWM IC. The higher component count and circuit complexity generally has a negative impact on the overall power supply reliability.
FIG. 1 illustrates a conventional non-latching implementation of a soft-start reset mechanism in a PWM IC. The current source Is charges the soft-start capacitor C1 during start-up. When a fault condition or shutdown command is generated, Qd discharges C1. SEU events can inadvertently result in Qd being turned on and the quick discharge of C1, causing an interruption in normal operation of the power supply.
FIG. 2 illustrates a modification for improving the SEU response of the circuit of FIG. 1. The addition of the resistor R1 limits the discharge current through Qd. Therefore, during the SEU event, C1 will become only partially discharged thus preventing interruption of the power supply operation and allowing faster recovery from the SEU event. Since C1 is charged by the constant current source Is, R1 does not interfere with the desired charge duration of C1.
FIG. 3 illustrates a latching implementation of the soft-start reset mechanism in a PWM IC. The soft-start capacitor C1 is charged to a predetermined voltage through the resistor divider comprised of R1 and R2 during power supply start-up. When a fault condition or shutdown command is generated, the SCR is triggered and discharges C1 until the discharge current drops below the SCR's hold current. Thus, the discharge mechanism is latched until C1 is discharged to a predetermined level, at which point the discharge mechanism is reset and the power supply then re-starts. SEU events can inadvertently result in triggering of the SCR (or equivalent latching circuit), which then would force the near complete discharge of C1 and thus cause an interruption in normal operation of the power supply. Adding a resistor in series with the capacitor as in the previous example (FIG. 2) will not work in this case because the required value of resistance would be prohibitively large and thus cause interference with the desired charge duration of the capacitor C1.
The prior art circuit of FIG. 2, although helping to mitigate against SEUs, may not prevent the SEU discharge of the soft-start capacitor C1. In the circuit of FIG. 2, although the resistor R1 reduces the rate at which the capacitor discharges due to an SEU, it also reduces the rate at which the capacitor discharges in the event of an actual fault or shutdown, which is undesirable. It is desirable to provide a circuit which prevents SEUs from discharging the soft-start capacitor and which does not result in changing the capacitor charge duration in the event of an actual fault or shutdown.
It is also desirable to provide a circuit which prevents SEUs from discharging the soft-start capacitor and which also allows discharge of the capacitor in the event of actual faults or shutdown conditions.